This invention relates to optical cables and, more particularly, to the design of a dielectric strength member system.
Optical fibers are in widespread use today as transmission media because of their large bandwidth capabilities and small size. However, optical fibers are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent. Transmission degradation, which results from bending, generally takes the form of macrobending and/or microbending loss. Consequently, cable structures have been developed to protect the optical fibers in various situations. Additionally, optical cables use glass fibers as a communications medium rather than copper wires; and while glass fibers are relatively strong, care must be taken to avoid excessive tensile stress because they are quite thin and are not ductile. Moreover, the optical transmission characteristics (e.g., index of refraction) of glass change in response to the application of stress. Therefore, strength members are generally included in optical cables to receive most or all of the stress due to tensile loading before it can be transferred to the optical fibers.
An optical cable having excellent strength performance is described in U.S. Pat. No. 4,844,575 that issued to Kinard et al. on Jul. 4, 1989. This cable comprises one or more optical fibers that are disposed within a cylindrical plastic tube, and a pair of metallic rods that are positioned on diametrically opposite sides of the tube and extend along the length of the cable. Steel is a preferred strength member for an optical cable because its tensile stiffness is suitable for receiving axial loading and its compressive stiffness is suitable for inhibiting contraction of the cable. Moreover, the cross-section area of a steel strength member is relatively small in comparison with other materials so that it does not undesirably increase the overall diameter of the optical cable. Nevertheless, there has been a long felt need for an all-dielectric cable construction. Such a cable could be strung from building ducts to service distribution points, and would obviate the need for grounding connections at splice points that add to the cost of cable installations. Further, such a dielectric cable would decrease the probability of lightning strikes.
A dielectric optical cable having excellent strength performance is disclosed in U.S. Pat. No. 5,109,457 that issued to Panuska et al. on Apr. 28, 1992. In this patent, the metallic rods of the Kinard et al. patent are replaced with non-metallic rods for tensile and compressive stiffness, and non-metallic rovings for added tensile stiffness. The rods are made from E-glass fiber filaments that have been impregnated with epoxy, and the rovings are made from E-glass fiber filaments without epoxy. FIGS. 5 and 6, herein, show this all-dielectric optical cable in greater detail. The combination of rods and rovings provides excellent strength and flexibility in a relatively small-diameter cable; however, it is desirable to minimize the number of components in a strength member system to simplify its manufacture. However, increasing the diameter of the pair of rods to eliminate the rovings increases the overall diameter of the cable. Nevertheless, for ease of access to the optical fibers within the cable, it is still more desirable to use a pair of diametrically opposed strength rods rather than a larger number of smaller rods that are disposed around the circumference of the cable.
In addition to a smaller diameter cable, it is also desirable to provide one that is capable of being blown through an empty duct within another cable that is already installed in the ground. Such optical fiber cables should be sufficiently flexible to facilitate passage through the empty duct during blowing. Unfortunately, optical cables having diametrically opposed rods exhibit preferential bending in that they bend readily in the plane that passes through the rods, but do not bend readily in an orthogonal plane. Such preferential bending is undesirable during blown cable applications.
Accordingly, what is needed is a dielectric optical cable whose strength system includes diametrically opposed rods, but nevertheless exhibits reduced preferential bending. Additionally, it is desirable that the cable have a reduced outer diameter, suitable flexibility and equivalent tensile strength in comparison with similarly designed optical cables.
An optical fiber cable according to the present invention includes a core tube containing optical fibers, a plastic jacket that surrounds the core tube, and a pair of linearly extending, diametrically opposed dielectric rods that are at least partially embedded in the jacket. The rods have a compressive stiffness that is effective to inhibit substantial contraction of the cable and a tensile stiffness that is effective to receive a tensile load without substantial transfer of the tensile load to the optical fibers. Each rod is surrounded by a frictional adhesion coating that enables it to move locally within the jacket in response to compressive or flexural stress applied to the cable.
In a preferred embodiment of the invention, the use of aramid fibers is completely avoided within the optical cable, and each dielectric rod comprises packages of glass fibers that are embedded in epoxy. The frictional adhesion coating is selected to be relatively soft (i.e., a hardness that is less than 80 D on the Shore durometer scale), which provides a high coefficient of friction with the jacket. Advantageously, when such frictional adhesion materials are used, flexibility is increased and the thickness of the jacket adjacent to the rods can be as small as 0.76xc2x113 mm. (i.e., 30xc2x15 mils) without fear that the cable will split when subjected to local twisting.